Group III nitride semiconductor, e.g., GaN (gallium nitride) semiconductor, has been expected as a material for a next-generation semiconductor device. GaN semiconductor device has wider bandgap than Si (silicon), and can attain higher breakdown voltage and lower loss than a Si semiconductor device.
A GaN semiconductor transistor generally has an HEMT (High Electron Mobility Transistor) structure using two-dimensional electron gas (2 DEG) as a carrier. A normal HEMT is a normally-on transistor that becomes conductive even if voltage is not applied to a gate. Therefore, there arises a problem in which it is difficult to implement a normally-off transistor that becomes conductive only when voltage is applied to a gate.
A power supply circuit that processes vast amounts of power such as hundreds to one thousand volts is required to have a normally-off operation with an emphasis on safety. In light of this, a circuit structure attaining a normally-off operation by a cascade connection between a normally-on GaN semiconductor transistor and a normally-off Si transistor has been proposed.
A GaN semiconductor transistor has small avalanche resistance, so that it might be broken due to serge voltage. The circuit structure using the cascade connection also has a problem of deterioration in reliability, because it has low resistance to serge voltage.